Justification:
The estimated extent of occurrence (EOO = 4,500 km²) qualifies the species to be evaluated in a threatened category, and even though many of the known subpopulations occur within the protected areas network, the species is generally described as uncommon and it is believed that the populations not currently protected are of high concern due to the serious logging and clearing pressure on forest habitat. Moreover, also within the reserve boundaries deforestation is sometimes an ongoing problem, suggesting the possibility of a further decline in the extent and quality of the forests habitat, which represent a serious problem for this Acacia species which grows better in shady woodlands habitat. The species will need to be monitored over a longer period of time to make sure of the status and health of the known populations. The effectiveness of ex situ and in situ conservation measures should also be determined.

A. allenii has been described as uncommon in a collection made in 1989 in Corcovado National Park (Kernan #1036). The species was also reported to be occasional in the brushy pastures in the area around Golfito in the 1940s, when much primary forest was being cleared, and was totally absent from these pastures in the period 1965-1970 (Janzen 1974), but present along the forests edges. In forest habitat A. allenii can live around 25 years, reach 22 m height and it is able to resist light ground fires, whereas when the forest is cleared the species produces shrubby bushes that might live for 1-4 years and reach only 2-3 m height. Ants are less successful at clearing the litter when A. allenii is in this shrubby form and the resulting fires kill both the Acacia shrubs and the associated ant colonies (Janzen 1974).

A. allenii is a tree up to 25 m tall with widely spreading branches and which grows in primary and disturbed forests, heavily shaded understory in wet forests, in moist disturbed stream banks, at forests edge and along roadsides. In general it reacts fairly well to disturbances, recent landslides, disturbed stream banks and secondary successional areas after logging. A. allenii is a myrmecophyte species (plant that lives in association with a colony of ants and has a symbiotic relation with it) and is part of the group known as the swollen-thorn acacia species, associated with stinging ants (i.e. Psuedomyrmex ferruginea or Pseudomyrmex particeps, a rare ant, which appears to be associated exclusively with A. allenii). Seigler and Edinger (1995) stated that the narrow geographic range of A. allenii is typical of many of the ant-acacia species of Central America, and that probably reflects rather narrow tolerances to moisture and temperature (Janzen 1974).

Logging and clearing of remaining forests are the main threats for the species. Although a few large blocks of intact habitat still exist, most of the lowland forest is already quite fragmented and the remaining intact forests in this ecoregion are currently being logged and cleared for cattle production. The Osa Peninsula has suffered of high rates of deforestation, it is estimated that between 1980-1995 around 18,000 ha of forests have been cut down, an average of 1,176 ha/year (Lobo and Bolaños 2005). Also protected areas are exposed to logging pressure: Golfo Dulce Forest Reserve has suffered of high rates of deforestation and it is described as the area where deforestation has been more intense in the country in the last 15 years (about 1,000 ha per year; Lobo and Bolaños 2005).

There are no known conservation measures specifically for A. allenii, but the species is known to occur in some protected areas such as Corcovado National Park, Golfito National Wildlife Refuge, Piedras Blancas National Park, Golfo Dulce Forest Reserve, Los Santos Forest Reserve and Portalon National Wildlife Refuge. Samples of seed of A. allenii should be collected and stored as an ex situ conservation measure.